1. Field of the Invention
The invention relates to water sanitation and maintenance.
2. State of the Art
Swimming pools and other bodies of recreational water provide a source of entertainment for people all over the world. Whether used to cool off on a hot summer day or to swim laps for exercise, swimming pools are an important part of our culture. They offer a safe and clean environment for people, especially children, to enjoy the water even in landlocked, dry locations.
Although swimming pools have many positive characteristics, they can also be breeding grounds for bacteria and other living organisms including disease-causing microbes. Thus, in order to provide safe, sanitary, and aesthetically pleasing water, proper swimming pool maintenance must be undertaken.
The key factors in maintaining recreational water are disinfection, sanitation, oxidation, water balance, filtration, and circulation. Sanitation kills undesired living organisms in the water. Disinfection controls disease-causing microbes. Oxidation oxidizes organic and synthetic contaminants to benign forms. Water balance refers to the pH, alkalinity, and calcium water hardness. Filtration removes dead organisms and oxidized debris from the water. Circulation works with filtration to provide clean chemically treated water to all areas of the pool.
The most common agent for oxidation and sanitation is chlorine. When chlorine gas, Cl2, dissolves in water, it hydrolyzes according to the reaction:Cl2(g)+H2O(l)→HOCl(aq)+HCl(aq)The chlorine of the hydrochloric acid does not contribute to the sanitation. It is the hypochlorous acid, HOCl which actually kills the microbial bodies. The bactericidal power of hypochlorous acid is attributed to its ability to defuse through cell walls and reach the vital parts of the bacteria cells. Hypochlorous acid, being a weak acid, ionizes in water according to the reaction:HOCl(aq)⇄H+(aq)+OCl−(aq)According to Le Chatelier's Principle, this reversible reaction will move towards equilibrium when subjected to an external change. Thus, if a product is added to the system, the reaction will shift to the left and more reactant will form. If the amount of product is reduced, the reaction will shift to the right and more product will form. Thus, if the pH of the water is lowered, making the water more acidic and raising the H+ ion concentration; the equilibrium is forced to the left and more hypochlorous acid is present. Conversely, if the pH of the water is raised, the H+ ion concentration is lowered and the reaction is forced to the right; forming more OCl− ion. Consequently, any free chlorine or hypochlorite added to water will immediately distribute itself into HOCl and OCl− with the ratio for the two being controlled primarily by the pH value of the water.
As a bactericide, chlorine kills unwanted organisms in water including algae and is non-toxic to humans in low concentration. Traditionally, solid chlorine tablets, liquids, and various dry compounds of chlorine have been added to pool water to maintain the recommended concentration. However, this requires daily maintenance and is very time consuming.
Due to recent improvements in process control, chlorine generators have become a popular tool in pool management. Chlorine generators, also known as salt generators, electrolytically produce pure chlorine gas form chlorine salts added to the water. Commonly, table salt or sodium chloride is used as the chlorine source for these generators. For the oxidation and sanitation steps, one simply adds an amount of sodium chloride based on the size of the pool.
Chlorine generators thus offer residential and commercial pool owners and managers a safe convenient way for maintaining the proper chlorine concentration in their pools. However, problems may be encountered when using chlorine generators. For example, it may be difficult to maintain the recommended free chlorine concentration even though the correct amount of sodium chloride was used. This is due to the free chlorine being degraded by sunlight. Ultraviolet radiation from the sun greatly depletes free chlorine in a short period of time. To reduce this depletion, a stabilizing agent or so-called conditioner is commonly added to the pool water. Cyanuric acid is by far the most commonly used stabilizing agent. This chemical assists in keeping the sanitizer in the water by preventing or slowing down the ultraviolet degradation of chlorine. The ideal concentration of cyanuric acid in the water is between 30 to 50 ppm or parts per million. Present in this concentration, cyanuric acid will keep chlorine in water 8–10 times longer than in water with no cyanuric acid. Without cyanuric acid, chlorine loss is 90 percent in just 2 hours. However, cyanuric acid is accompanied with its own issues. It is an acid so it will lower the pH of the water. It is also very slow dissolving. It can take from 4 to 7 days to dissolve in a pool depending on circulation, water balance, and water temperature. If non-dissolved cyanuric acid is left in contact with the pool vessel surface for a prolonged period of time, it can etch plaster, degrade or stretch vinyl liners, and corrode metals which can stain the swimming pool vessel surface. Further, if cyanuric acid is added through a pool's surface skimmer, as is commonly done, non-dissolved cyanuric acid can harm the metal components in the pool's plumbing and filtration system.
There are also problems encountered relating to fouling of the electrolytic plates in chlorine generators. Fouling refers to chemical buildup on the plates. The electrolytic plates are subject to shortened life spans if suntan oil, grease, soap scum, or other hydrocarbon-based synthetic personal care chemicals coat or foul the plates. The efficiency of chlorine generators are reduced if the plates become dirty. The use of an enzyme based product added to the water will help prevent the plates from getting fouled. Regular use of an enzyme in the water will also help reduce the amount of grease and oil present in the pool water.
Chlorine generator plates can also be fouled by calcium and magnesium precipitates building up on the plates. The most common buildup is calcium carbonate or so-called scale. Extreme scale buildup can bridge the distance between plates. This can cause either a short circuit or a burned area on the plate. The addition of ethylenediaminetetraacetic acid, or EDTA. is one common way of controlling scale. EDTA's main function is to prevent stains by chelating metal ions. When filling a pool for the first time or whenever a significant amount of water is added there is a risk of metallic stains. Thus, a chelating agent is commonly added while filling or soon after filling to prevent staining.
There are several other factors crucial to maintaining a safe, clean pool. For example, not only is the concentration of chlorine important for effective disinfection but also the efficacy of the chlorine. Algae greatly reduce the efficiency of chlorine in combating disease causing microbes. With algae present, the free chlorine is tied up fighting the algae and its effectiveness against other organisms is reduced. Orthophosphates are the limiting nutrient for algae growth. All phosphates in the water can degrade into orthophosphates. Thus, by reducing the phosphate concentration, the amount of algae that is capable of growing is reduced. Ideally, the phosphate concentration is kept below 100 parts per billion or ppb and most preferably kept below 50 ppb. At this concentration, only a small amount of algae is able to grow. To kill such a small population of algae, only a small amount of chlorine is needed. Thus, the remaining chlorine is free to combat other undesired organisms and the overall efficacy of the free chlorine is raised. To this end, phosphate removers are commonly added to recreational water. Phosphate removers typically include lanthanum salts and cause phosphates to precipitate out allowing them to be easily removed by filtration or vacuuming. For example, PHOSfree® of Natural Chemistry, Inc. utilizes lanthanum sulfate.
As discussed above, water balance, and especially the pH of the water, is another important factor in determining the efficacy of the free chlorine. When chlorine is added to water it forms HOCl (hypochlorous acid) which then dissociates into OCl− (hypochlorite ion). HOCl is a very powerful killing form of chlorine in water. OCl− is a rather weak disinfectant and is about 80 times less effective than HOCl. The percentage of each of these forms is determined by the pH of the water. At a pH of about 7.5, there is about 55 percent of the chlorine in the HOCl form and about 45 percent in the OCl− form. At a pH of about 8.0, there is about 28 percent in the HOCl form and about 72 percent in the weaker OCl− form. Therefore keeping the pH in the proper range renders the chlorine more powerful and efficient. In order to maintain the pH within the desired range, pH buffers are commonly added to the water.
Maintaining the proper balance between all these chemicals is no easy task for pool owners and managers. Pool owners and managers constantly over or under dose pool chemicals because most doses are calculated in ppm or mg/L (milligrams per liter) both of which are a weight-to-weight expression and they commonly use measuring cups, to measure out the chemicals, which is a volume measurement. Because of different densities, one cup of a dry chemical is not always 8 oz. dry weight. Other pool owners and managers simply guess at the weight based on a proportion of the container it is in. For instance, ⅕th of a 5-pound container is about one pound.
Further, even adding the correct sodium chloride is not always an easy task. Because salt is a commodity and cheap, many pool dealers do not sell it in their stores. Many dealers send customers to home and building supply chains. Thus, there is a chance that the customer will buy the wrong kind of salt. For example, salt can be purchased with hardening agents (e.g. sodium hexametaphosphate or SHMP), cleansers (e.g. citric acid based), free-flowing/anti-caking agents (e.g. Yellow Prussiate of Soda or YPS), iron removal agents, detergents, and surfactants. While not dangerous, adding the wrong salt could reduce the life of the chlorine generator. It may also cause some staining and discoloration of the pool vessel.
In view of all the variables in maintaining safe clean swimming pool water, there is a need for simplifying pool management. Specifically, there is a need for reducing the number of steps required to effectively maintain the water in swimming pools chlorinated with chlorine generators.